The present invention relates to a protective sheath which houses a prestressed concrete (hereinafter "PC") cable such as PC steel wire, PC twisted steel wire or PC steel rod that is used as tension wire in post-tension techniques or pretension techniques during construction of prestressed concrete structures, and which protects the PC cable by preventing it from coming into direct contact with the concrete.
Conventionally, as means for rust-proofing and preventing deterioration of the PC cables used in the construction of this kind of prestressed concrete structure, a steel sheath is generally used so that the PC cable does not come into direct contact with the concrete. On the other hand, instead of this steel sheath, the use of a synthetic resin sheath has begun to be proposed.
With regard to the former steel sheath which has been generally used heretofore, there is a type in which annular irregularities are formed in the sheath (node application) and a type in which helical irregularities are formed (wave application). In both the types, the entire pipe wall is of uniform thickness. With regard to the latter synthetic resin sheath currently being proposed, its form is the same as that of the steel sheath, its wall thickness is entirely uniform, and its uneven wave shape is formed in simple arc-shaped waves (for example, see Japanese Utility Model Unexamined Publication No. Hei 6-21953).
With respect to the steel sheath, since it comes into direct contact with the concrete, it tends to be corroded by salinity contained in or adhering to sands (fine aggregate) contained in the concrete. In particular, there is a problem that rust and deterioration tend to occur due to acid rain which penetrates through minute cracks in the concrete. Moreover, salt damage tends to occur in structures located in the sea, on the sea, or within 100 m of the coastline, and salt damage countermeasures have become a major issue. On the other hand, with respect to the synthetic resin sheath, there is a problem that countermeasures must be developed for the low compression deformation resistance in the circumferential and axial directions, and low abrasion resistance, which are characteristics of synthetic resin.